Rabastan is Regulus' best friend.
There's a two year difference between them but they grew up together, the Lestranges and the Blacks constantly calling on each other. They called on the Malfoys too, but there was only Lucius and he was so much older than Regulus that he would never play games with the little Black.
Not that either Regulus or Rabastan are loud troublemakers. Unlike Rodolphus, Rabastan is far more reserved and quiet. He is introverted in the same sort of way that Regulus is. Part of Regulus' quiet manner is his raising, of course, and having Sirius for an older brother. Regulus was born the younger son and he had always held a place in the shadow of the heir. Then there are the reserved manners of polite company that Regulus found far less burdensome than his brother and almost as though to not be like Sirius, to be better, to please his parents more, Regulus acts in a way as radically opposing Sirius as he possibly can.
Rabastan never needed quite so much encouragement from circumstances. He is serious by nature and he was drawn in by both Regulus' similar quiet as well as his more uplifted inner core. Regulus is far more idealistic that Rabastan, far more passionate. But they compliment each other nicely, finding balance between themselves.
Regulus found his outlet in quidditch before he entered Antonin Dolohov's training. Rabastan, however, kept mostly to himself and the library, even when Regulus came to Hogwarts, until he was admitted to the Academy. There he blossomed in a mature sort of way. Rabastan is good at fighting and he is decent at teaching. There is a precision in everything he does while Regulus is more smooth and lithe, a silken handkerchief and a fine wine as compared to Rabastan's silver dagger and rich brandy.
Together they complete a set, a decorative piece in the old, elegant style. Antonin likes to put them together as a fighting pair. Most would consider this an odd choice but it makes perfect sense to everyone who knows them. They can interpret each other well. Regulus works well with many people but Rabastan is harder to place and together they provide an effective, solid pair, especially on broom.
This intimacy comes from many things. From growing up together to the hours Rabastan spent tutoring Regulus to do things like ride his broom or pass his first month of transfiguration. They still take walks together and end up sitting on the edge of a pond or a stream, Rabastan drawing in the dirt with a stick and Regulus lying back on the grass, both hands under his head, staring up at the sky. Sometimes they do not speak, preferring to soak in the day, the world around them. They each interpret it in different ways – Rabastan in details and Regulus in splotches of color and feeling. Sometimes they will share their observations and laugh at each other's peculiarities but this only gives them better insight into each other.
Regulus enjoys flying a lot more than Rabastan but the young Lestrange will give in from time to time and go soaring with Regulus into the large blue expanse of the sky, brushing the bottoms of clouds and laughing into the wind. "It feels free doesn't it?" Regulus asks, flipping his broom. "You can be anything you want up here! Even a bird if you wish."
Rabastan nods, allowing himself to relax. Regulus allows him to calm his nerves, get rid of that snappiness that threatens to break loose sometimes. He knows he can't let it win or he will end up hysterical like his brother and Rabastan does not want that.
They share dreams of the future with each other. Rabastan's are more vague. He wishes to win respect in this war for his talents as a fighter. Regulus wants the same thing but in more boyish, glorified terms. He lives for the Cause and his family. Rabastan does the same but in a more withheld sort of way.
Their friends tease them sometimes, calling them the "love birds." Neither of them minds particularly. "Love comes in many shapes, does it not, gentlemen?" Regulus says softly one night while their battle squad is sitting around a spontaneously decided on bonfire and eats smores.
Avery, who had started this round of playful joking, shrugs. "You two just look very snug over there."
Rabastan smirks and puts one arm around Regulus as if to goad them on purpose.
"I think he's right. Love is…it's in everything that's beautiful and right. In everything that is worth fighting for," Anatole Bonfante says so seriously that Barty snickers and Severus rolls his eyes. Severus receives an elbow in the side for that from Evan and Barty – an exasperated glare.
"Love is magic," Evan says softly and he meets Regulus' eyes.
"Magic is love," Regulus echoes and feels Rabastan's hand tighten on his shoulder.
"I'll drink to that!" Wilkes interjects happily and holds up a bottle of firewhisky. They laugh and drink and the war seems like a million miles and years away in that moment.
When Regulus dies, Rabastan feels like something more than a person has gone out of his life. He feels like the smoothness to everything is gone, like he can suddenly feel all the sharp edges he had only seen before. It makes it easier to keep fighting and living. It makes it easier to follow Rodolphus and Bellatrix into the hideout of Frank and Alice Longbottom, to cast the necessary curses and to watch his brother and sister-in-law do most of the dirty work as he stands guard. Regulus had never liked revenge but Rabastan saw its utility. Now that Regulus is gone, he feels that utility as well. Barty, who is with them, seems indifferent at first, but the pressure of the scene takes hold of even him finally and he begins to wince and squirm. His wand hand droops and he looks questioningly at Rabastan, as though to ask if this really has to be done.
"They don't know anything," Barty mouths.
"For Regulus?" Rabastan asks. He knows, deep inside, that Regulus might not have wanted this. But he isn't the only casualty in this war.
Barty confirms this a moment later as his hand stills and his shaking subsides and calms into resolve. "For everyone."
Rabastan had always thought that if he could bring one person back from the dead it would be Regulus. But as he stands before the Wizengamot he thinks that perhaps it is better that Regulus did not live to suffer this humiliation. That does not make missing him any easier.
Regulus was Rabastan's best friend.